Implement a composed primitive Shutdown

[tisonkun]

#[derive(Debug, Clone)]
pub struct Shutdown {
    latch: Arc<Latch>,
    wg: WaitGroup,
}

impl Shutdown {
    pub fn new() -> Self {
        Shutdown {
            latch: Arc::new(Latch::new(1)),
            wg: WaitGroup::new(),
        }
    }

    pub fn shutdown(&self) {
        self.latch.count_down();
    }

    pub async fn await_shutdown(self) {
        self.wg.await;
    }
}

This is used in ScopeDB to synchronize the shutdown of scheduled actions.